The present invention relates to integrated circuits and, more specifically, to joining integrated circuits together.
Controlled-collapse chip connection (C4) is a means of connecting integrated circuit (IC) chips to substrates in electronic packages. C4 is known as a flip-chip technology, in which the interconnections are small solder balls on the bottom side chip surface. C4 technology represents one of the highest density schemes known in the art for chip interconnections. Historically, the PbSn (lead-tin) solder for the formation of the solder ball was evaporated through a metal mask. In the 1990's, electrochemical fabrication of C4 interconnections was introduced. Electroplating is more extendible than evaporation to small C4-pad dimensions, closer pad spacing, larger wafers, and lower-melting solders (which have a higher content of tin (Sn)).
In general, the top layers of an integrated circuit (IC) chip are wiring levels, separated by insulating layers of dielectric material that provide input/output for the device. In C4 structures, the chip wiring is terminated by a plurality of metal films that form the ball-limiting metallurgy (BLM), which is also referred to as under-bump metallurgy (UBM). The ball-limiting metallurgy defines the size of the solder bump after reflow, provides a surface that is wettable by the solder, and that reacts with the solder to provide good adhesion and acceptable reliability under mechanical and heat stress. The BLM also serves as a barrier between the integrated-circuit device and the metals in the interconnection.
FIGS. 1A and 1B are a typical implementation of the C4 manufacturing process. In FIG. 1A an integrated circuit (IC) 100 formed on a base material 102 (for example, silicon) has a solder ball 108 formed for subsequent attachment to a contact pad 112 (see FIG. 1B) on a carrier 114. A BLM 106 constricts the solder flow and aids in the formation of the solder ball 108 (which is formed by reflowing a deposit of solder paste), and serves as a wettable surface and contact for an underlying contact 110 for the IC 100. A passivation layer 104, typically a polymer dielectric, insulates the IC 100, and supports the BLM 106. In FIG. 1B the IC 100 is attached to the contact pad 112 on the carrier 114, by reflowing the solder ball 108. Solder flow is restricted on the carrier 114 by solder dams 116, which outline and define the contact pad 112. A secondary reflow is employed to attach the IC 100 to the contact pad 112 on the carrier 114.
Certain state and federal regulations have limited or eliminated the use of lead based solder. One approach to complying with these regulations includes utilizing tin (Sn) based lead-free solders. Sn has a tetragonal crystal symmetry that exhibits anisotropic properties such related to elastic constants and diffusion of solute atoms through the Sn. In first orientation of the Sn particles, elements that form the contact pad 112 (e.g., copper (Cu) or nickel (Ni) diffuse under an electric field at a rate thousands of times slower than when the particles in a second orientation perpendicular to that of the first orientation. Solder balls 108 having the first orientation (e.g., with low diffusion) exhibit a slow/controllable failure of joint known as Mode 1 electromigration failure herein. In this mode, the failure has a formation of voids near the surface of the BLM 106. For solder balls having the second orientation (e.g., perpendicular to the orientation for Mode 1), a failure known as Mode 2 electromigration failure herein occurs. Mode 2 failure is characterized by the movement of intermetallics from the carrier 114 to the BLM 106 or vice-versa. The possibility of mode 2 failures make tin based lead-free solder balls 108 unfit for the high-end applications requiring long life (e.g., 100 k Hr) at elevated temperatures (e.g. 100 C).
Several efforts are in progress to make the ball 108 an agglomerate of large number of randomly oriented grains to reduce the effective mass flow of Cu or Ni perpendicular to the BLM 106. However, due to the small size of the ball 108, the number of grains per ball 108 is limited to less than 5.